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Polypseudorotaxanes, which feature α-cyclodextrin (α-CD) rings that traverse a poly(ethylene glycol) (PEG) chain, are gaining traction as potential building blocks for molecular machines. However, the molecular dynamics governing their behavior have not been well understood until now. Researchers have recently leveraged fast-scanning atomic force microscopy (FS-AFM) to observe the movement of α-CD rings along a PEG chain. This advancement solidifies FS-AFM as an effective method for studying supramolecular polymers and could facilitate the development of more efficient molecular motors.
Consider a minute locomotive navigating a track, capable of self-propulsion without any external force. This concept is at the core of molecular motors—complex systems that hold the promise for advanced materials, precise drug delivery, and the fabrication of nanoscale robotics.
Driven by the inspiration from molecular systems found in nature, scientists have been crafting synthetic counterparts since the inaugural synthetic molecular machine was made in 1994. The field has seen remarkable growth, leading to the 2016 Nobel Prize in Chemistry awarded for significant advancements in molecular machine technology. Among potential candidates is the polypseudorotaxane structure, where a PEG polymer chain is threaded through multiple α-CD rings. In solution, these rings self-assemble onto the PEG chain and exhibit movement along it. Until now, the specific structural alterations accompanying this movement have remained elusive.
Recently, a research team from the Japan Advanced Institute of Science and Technology (JAIST), led by Associate Professor Ken-ichi Shinohara, successfully captured the dynamic shuttling of α-CD rings in real-time along the PEG chain. Their work illuminated previously unclear localized structural changes. Utilizing the specialized FS-AFM, the researchers produced real-time imagery of the α-CD rings in motion along the PEG chain, as outlined in their study published in Macromolecules on March 4, 2025. This approach introduces a new avenue for analyzing the structure of supramolecular polymers—one that could enhance the design of sophisticated molecular machines.
“Despite the common usage of PEG@α-CD polypseudorotaxane, the specific structural modifications occurring during the shuttling of α-CD rings along the polymer chain have not been well characterized. By revealing its structure at the solid-liquid interface, our findings should contribute to the advancement of synthetic polymer motors that are driven by thermal fluctuations,” states Dr. Shinohara.
In their methodology, the researchers created the polypseudorotaxane by combining PEG100k with α-CD in an aqueous solution, allowing the mixture to sit for over six hours. This resulted in the formation of a white solid, which was then studied using FS-AFM in a 15 millimolar potassium chloride aqueous environment. Unlike traditional optical microscopy, AFM employs an ultra-sharp tip on a delicate lever to scan surfaces, enabling it to capture nanoscale structures and produce high-resolution images.
When analyzing the PEG100k chain alone, it was found to possess a highly flexible, dumbbell-like shape, with globular ends. This flexibility endowed it with spring-like characteristics, allowing for both expansion and contraction. Notably, when in a relaxed state, the chain measured approximately 48.1 nm on average, significantly shorter than its fully extended length of 790 nm. Upon the introduction of α-CD rings, the chain’s flexibility diminished. Imaging the PEG100k@α-CD polypseudorotaxane revealed a more rigid structure averaging 499.6 nm in length, complete with end-cap formations that prevented the α-CD rings from detaching. Remarkably, despite its reduced flexibility, the chain maintained its spring-like motion, with α-CD rings actively shuttling along its length.
“We found that the polypseudorotaxane exhibited both shrinking and extending movements driven by the motion of α-CD rings along the polymer chain. Primarily, these changes were observable in the exposed, self-shrinking segments of the PEG, where the α-CD rings caused repeated expansion and contraction,” Dr. Shinohara elaborates. Complementary molecular dynamics simulations corroborated these observations, successfully mirroring the movements documented during the FS-AFM experiments.
While the realization of fully operational molecular machines is still a distant objective, this study serves as a fundamental step toward comprehending molecular dynamics in supramolecular structures. “FS-AFM is a valuable tool for studying supramolecular materials, especially in situations where conventional spectroscopic techniques might not adequately provide structural insights,” remarks Dr. Shinohara. These discoveries are positioned to pave the way toward energy-efficient molecular motors capable of exploiting thermal energy at room temperature for controlled movement.
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